Some communication systems use an adoptive array antenna system for appropriately controlling directionality of an array antenna comprising a plurality of antennas. At surrounding base stations located around a base station, when signals are transmitted to a mobile phone using the adoptive array antenna system, null steering is performed with respect to a transmission directionality of an array antenna so as to direct nulls to the mobile phone in order to suppress interference with a mobile phone communicating with the base station. Because a number of nulls that can be set in null steering depends on a number of antennas in an array antenna, it may be impossible to direct nulls to the mobile phone to which they are required to be directed at surrounding base stations.